Over the past couple of days I've noticed, when I get up in the morning, that the level of water in the toilet bowl is about half the level it usually is (this has happened once or twice before, but always when there were high winds outside, which there aren't now). Also, when I flush the toilet, the taps on both the sink and tub start to run. Once the toilet tank finishes filling, the sink and tub taps stop running. What's going on?